On ignition devices using an ignition coil, a high discharge voltage is produced or induced in a secondary coil by interrupting primary current at predetermined ignition timing after having energized the primary current to the primary coil, thus generating an electric discharge between the opposing electrodes of a spark plug connected to the secondary coil. Basically, the discharge voltage and discharge energy induced in the secondary coil correlates with the primary coil energization time (see FIG. 6).
The aforementioned primary coil energization time, which influences the discharge energy, is generally determined by engine revolution speed. Conventionally, the lower the engine speed, the longer the energization time. However, Patent document 1 teaches that the energization time is lengthened in a high load region, whereas the energization time is shortened in a low load region.
As one of abnormal combustion conditions of an internal combustion engine, pre-ignition in which combustion starts before ignition timing is generally known. A so-called super-knock phenomenon is a type of pre-ignition. Such pre-ignition tends to occur in a low-speed high-load region in either of a natural-aspirated internal combustion engine and a supercharger-equipped internal combustion engine. Assuming that the pre-ignition is occurring, the in-cylinder gas density at original ignition timing becomes high. In such a situation, even when primary current is interrupted and thus a high voltage is produced, a so-called no-discharge state may possibly occur without any electric discharge between the electrodes of a spark plug. FIG. 11 shows comparison between an in-cylinder pressure change during normal combustion (a) in which ignition combustion has been achieved normally at ignition timing (original timing) and an in-cylinder pressure change during the occurrence of pre-ignition (b). As appreciated from these characteristics, during the occurrence of pre-ignition a high in-cylinder pressure has already been produced at the original ignition timing, and thus there is a possibility for no-discharge to occur without any desired electric discharge. In the case of such a no-discharge state, a coil generated maximum voltage tends to be applied to the spark plug. An electric discharge through the insulator of the spark plug occurs, and hence there is a possibility for the spark plug to be damaged. By the way, the coil generated maximum voltage during the no-discharge also correlates with the primary coil energization time (see FIG. 8).
Therefore, suppose that the energization time is lengthened in a high-load region as disclosed in the Patent document 1. In the case of no-discharge during the occurrence of pre-ignition, there is a higher possibility for the spark plug to be damaged.
In contrast, suppose that a normal discharge energy is set lower in order to avoid the spark plug from being damaged even in the presence of no-discharge. In such a case, it is impossible to achieve more certain ignition in a so-called hardly-flammable region, such as an operating region in which a large amount of exhaust gas recirculation (EGR) is performed, a lean-burn operating region, a Miller-cycle combustion operating region and the like.